In spinal fusion, two or more vertebrae are joined by a fusion material placed between the vertebrae. Once fusion is complete, the fusion material immobilizes the vertebrae. Spinal fusion is used primarily to treat pain caused by abnormal motion of the vertebrae. Anterior lumbar interbody fusion (ALIF) is a spinal fusion technique that can be used for treating degenerative discs from an anterior approach. The anterior approach allows access to the interbody space with minimal damage to the posterior musculature, while allowing full decompression of the diseased disc. During an ALIF procedure, an interbody device is inserted within the intervertebral body space. In many cases, the interbody is a rigid body with a central opening. The opening may be filled with bone graft material, such as an autograft or allograft material. The objective of interbody fusion is to fuse the central graft material to the cranial and caudal endplates, creating a rigid boney union between motion segments.
Many known interbody designs utilize a rigidly constrained fixation, in which anchors, such as bone screws, are rigidly set in the interbodies and in the bone. The anchor screws are constrained in the sense that they are fixed and cannot move relative to the interbody. This rigid construct is intended to provide stability to the implant. At the same time, a rigid construct will stress-shield the graft material. That is, the interbodies and anchor screws form a rigid frame around the graft material that absorbs axial loads from the vertebrae being fused together, thereby shielding the graft material from axial load.